Glazes, whether raw glazes or spar glazes or frit containing glazes, in general are mixtures of inorganic compounds present in defined mixing ratios, which mixtures are applied onto ceramic compositions in the form of a glaze slip and during glaze baking are melted to form an enamel. Glazes for use at baking temperatures of less than 1200.degree. C. mostly consist of ceramic frits and additives. The production of said frits depends on the use of water-soluble raw materials such as alkali compounds, boron compounds or lead compounds, which, in the course of the melting process employed in the production, are converted to resistant glasses.
For glazes and frits which form ceramic glasses, more specifically, there are used oxides sin combination with silicon dioxide. These oxides are B.sub.2 O.sub.3, Al.sub.2 O.sub.3, K.sub.2 O, Na.sub.2 O, Li.sub.2 O, CaO, MgO, SrO.sub.2, BaO, ZnO, PbO, SnO.sub.2 and ZrO.sub.2. The individual oxides are intended to serve different purposes in the glaze. PbO, B.sub.2 O.sub.3 and the alkalineoxides are considered to be typical fluxes. SnO.sub.2 and ZrO.sub.2 are added to impart a white opaqueness to the glaze.
The frits employed in the glazes are prepared by melting the individual components in a melt process to form multicomponent silicates. These melts are solidified by quenching them in water or between cooled rollers. More particularly, cooling between cooling rollers is employed in continuous melting processes or with frits having an insufficient amount of silicate bonding, especially when the compounds of sodium, potassium or boron are present in higher proportions. The melting process for producing frits is carried out either periodically in rotary melting furnaces or in tank furnaces by use of a continuous melting procedure. In all of these production processes the melter units are are lined with refractory ceramic materials. The mixtures, in order to be provided with a sufficient flowability, must be heated above their melting point. Depending on the viscosity of these glasses there is an attack against the lining of the melting unit which becomes noticeable as wear and requires the ceramic lining materials to be frequently replaced. For quenching the liquid melt a large amount of water is necessary.
If, in raw glazes or spar glazes or even in frits, zinc oxide or zinc carbonate is used, high losses due to sublimation occur, especially where baking is conducted under reducing conditions. Apart from the undesirable loss of zinc, the sublimation causes disturbing deposits in the furnace which deposits have to be removed periodically with considerable expenditure ofwork. Therefore, attempts have been made in the art to keep the zinc oxide content in glazes as low as possible or even to completely avoid the use of zinc oxide, thereby to reduce or avoid the undesirable consequences of the sublimation.